Acoustic liners and abradable liners employed in front and rear fan casings of gas turbine engines. Acoustic liners are typically fabricated using a GRP (Glass Reinforced Plastic) or aluminium perforate skin, aluminium honeycomb and a GRP backing skin. Abradable liners can have a similar construction, but typically have an abradable layer to form a track for the tips of the engine fan blades. These components are bonded together to form the assembly and the assembly is then either bonded or bolted to the casing.
Successful bonding of the assembly to the casing requires the adhesive to be cured under a pressure usually recommended by its manufacturer. This pressure is applied in order to ensure that (i) the entire bonding areas on the surfaces of the mating parts are in contact with the adhesive and (ii) the entrapment of voids and the entrapment volatile gases discharged during the curing of the adhesive are mitigated or eliminated.
Accordingly, “vacuum bagging” may be used to hold the liners at predetermined positions around the curvature of the sides of the fan case and to consolidate the components during the curing of the adhesive. Vacuum bagging can provide any additional pressure that may be needed to bend or conform the relatively flexible liners to the curvature of the fan case.
However vacuum bagging can have the following disadvantages:    When a relatively small component such as an acoustic or abradable liner is bonded to a relatively larger component such as a fan casing, the entire structure of the larger component needs to be completely enclosed by the vacuum bag in order to obtain the necessary vacuum pressure for the consolidation of the components. Thus a considerable amount of vacuum bagging materials e.g. release fabric, breather, perforated film and polyethylene film are required even when the area of interest for the bonding may be much smaller. This can increase the cost of the bonding process.    Usually the polyethylene film used to create the vacuum seal is about 0.15 to 0.3 mm thick. In some cases when one of the components to be bonded is relatively stiff and its curvature does not match that of the corresponding component, the film may not be strong enough to push the mating components together during the curing. In such situations, the polyethylene film stretches or tears instead of performing the consolidation as expected. The strength and the in-service performance of the bonded structure can thus be significantly reduced.    The materials used for the vacuum bagging process care typically used only once before they are disposed. This also increases the cost of the process.
To address these issues, mechanical clamps and fixtures can be used to bend or conform liners to the curvature of the fan casing while the adhesive is curing. However, the assembly of mechanical fixtures is usually cumbersome and time consuming, and increases the risk of producing unintended inflexions in the liner. Also it can be difficult to ensure that equal consolidating pressures are applied to the various locations where the clamps are applied. As a result, there is a high risk that too much (or too little) pressure is applied at specific locations. When too much pressure is applied, the risk of squeezing too much adhesive out of the bonding area increases. In contrast, when too little pressure is applied, the risk of the entrapment of voids and volatile gases in the adhesive region increases significantly.
Another option is to use autoclaving equipment to apply consolidating pressures, but again complete encapsulation of the components is required.